Biomedical Engineering Reference
In-Depth Information
FIGURE 2-36
Measuring range
using a PSD-based
sensor.
(a) Operational
principles.
(b) Cutaway view of
PSD. (Brooker 2008.)
Though the current generated by the PSD depends on the intensity of the incident
radiation, the ratio of the two currents does not, so the distance to the target can be
determined with good accuracy. The PSD operates by exploiting the change in resistance
of a doped silicon semiconductor, which is proportional to the intensity of the incident
light. The device is fabricated from a thin slab of high-resistance silicon with the top
and bottom layers doped p and n
+
, respectively. Two electrodes, A and B, are placed
at opposite ends of the upper layer, and a common electrode, C, covers the bottom. The
distance between the upper electrodes is
D
, and the corresponding resistance is
R
D
.
If the beam strikes the PSD at a distance
x
from electrode A, the resistance between
that point and the electrode is
R
x
, and a photocurrent,
I
o
, proportional to the intensity of
the light, will flow. The amount that flows to the electrodes A and B will be proportional
to the relative distances from the incident beam; therefore,
I
o
R
D
−
R
x
R
D
I
o
D
−
x
D
I
A
=
=
(2.27)
and
R
x
R
D
=
x
D
I
B
=
I
o
I
o
(2.28)
Taking the ratio of the two currents
I
A
and
I
B
I
A
I
B
=
D
x
−
S
=
1
(2.29)
The distance,
x
, can then be written as
D
S
+
1
x
=
(2.30)
Substituting into (2.26) gives
2
L
B
D
S
+
1
1
=
k
S
+
1
L
o
=
f
(2.31)
S
−
S
−
1
where
k
is the module geometrical constant. Therefore,
L
o
, the distance from the sensor
to the target, can be determined in terms of the ratio of the two currents out of the PSD
(Fraden, 2003).
